A mask pattern forming method which can form desired resist patterns accurately and stably on a substrate material like SiC having transparency to light. The method comprising forming a light reflective film on a semiconductor substrate having transparency to light; forming a photo-resist on the light reflective film; projecting exposure light through a master mask onto a focal position adjusted based on light reflected by the light reflective film when projecting focus detecting light onto the semiconductor substrate, thereby exposing the photo-resist; and removing exposed portions or portions other than the exposed portions of the photo-resist so as to pattern the photo-resist.